Carlton insisted that the split was amicable and appeared to back Lord Alli's claim that he was leaving because he wanted to spend more time helping the Labour Party to fight the next election. However, insiders said that Lord Alli had refused to cut his political commitments, which were taking up a quarter of his time, and devote all his energies to the company.

Friends of Lord Alli, who ran Carlton's programme-making arm, claimed that he had been planning to leave for months. But that account does no fit in with the bold plans that Lord Alli, who spearheaded the campaign to lower the age of consent for homosexuals, has repeatedly outlined for Carlton.

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Only three weeks ago he told the FT that he believed his future was tied up with Carlton. There were suggestions yesterday, denied by Carlton, that Lord Alli had been hired principally for his close links with Labour. Mr Mandelson planned much of Labour's last election campaign at Lord Alli's house in Kent and the peer advises the prime minister on "youth" matters.

When Carlton paid £15 million for Planet 24, cynics claimed it was far too much for a company - founded by Lord Alli with his boyfriend, Charlie Parsons, and Bob Geldof - that made only one major programme, Channel 4's now struggling Big Breakfast.

However, insiders claim that Carlton's real target was Lord Alli and his political influence, as much as the company itself. Carlton had been anxious to secure Lord Alli's political connections before the company's planned merger with United News and Media.

Lord Alli initially antagonised many colleagues by installing himself and his 18-strong team over an entire floor of Carlton's building in Covent Garden. There were further ructions when he clashed with Steve Hewlett, a well respected former Channel 4 executive who was recruited to salvage Carlton's battered reputation as a programme maker.

Lord Alli will continue as executive producer of Carlton's Survivor, a castaway gameshow, which has proved a huge hit in America and will be screened in Britain next year. Ironically, it is likely to make more than £30 million in foreign sales for its owners, Lord Alli and Parsons, but not for Carlton because the programme was not part of its acquisition of Planet 24.